Both McDonald's and Burger King's hamburgers and French fries were low in the element, at about 20 mcg for the burgers and 3 mcg for the fries, Sun Lee, MD, of Boston University, and colleagues reported at the American Association of Clinical Endocrinologists meeting here.

The recommended daily amount of iodine for adults in the U.S. is 150 mcg.

"The amount of iodine was surprisingly small except in items like milk or fish, which we know are generally high in iodine," Lee said at a press briefing here.

Recent studies have shown an increasing problem with iodine deficiency in the U.S., Lee said. The compound is essential for hormone development, and is also important in pregnant and lactating women for normal fetal and neonatal neurodevelopment.

Burger King has endorsed the use of iodized salt in its food, Lee said, but McDonald's does not.

So the researchers decided to investigate whether there were differences in the iodine content between the two. They assessed two restaurants in each chain in the Boston area, and decided to test seven categories of food at both.

They analyzed two of each food item, measuring iodine content spectrophotometrically.

The researchers found that iodine varied from as low as 2.1 mcg in Burger King chicken tenders to as high as 163.7 mcg in a McDonald's vanilla shake.

Total iodine content for McDonald's items was as follows:

Big Mac with cheese: 16.7 mcg

Small French fries: 2.7 mcg

Fish sandwich: 69.9 mcg

Chicken sandwich: 5.3 mcg

Happy Meal hamburger: 4.3 mcg

Chicken nuggets: 3 mcg

Vanilla shake: 163.7 mcg

Total iodine content for Burger King items:

Whopper with cheese: 25.8 mcg

Small French fries: 4.3 mcg

Fish sandwich: 43.5 mcg

Chicken sandwich: 163.3 mcg

Kids meal hamburger: 3.9 mcg

Chicken tenders: 2.1 mcg

Vanilla shake: 147.8 mcg

Lee said it was not surprising that milkshakes and fish sandwiches had higher iodine contents because these foods are generally known to contain more iodine.

What was surprising, Lee said, was the Burger King chicken sandwich's high iodine content. But further analyses revealed its abundance was likely due to high iodine content in the bread from iodate used as a dough conditioner -- not from the chicken patty itself.

The researchers concluded that despite the difference in the use of iodized salt in food preparation, the iodine contents "appear to be similar between comparable items of McDonald's and Burger King."

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